This invention relates to an induction system for use in a multi-cylinder engine, more particularly to an engine induction system for use in an engine having a plurality of rows of cylinders in which intake runners extending from respective cylinders in each cylinder row are connected convergedly to a convergent portion.
Conventionally, various methods have been proposed for increasing the charging efficiency of an induction system so as to obtain higher power output of an engine. There is known a method for supercharging the intake air into cylinders by utilizing dynamic effects of induction such as resonance effect and inertia effect in the induction system as one of those methods.
For example, Unexamined Japanese Patent Publication No. 62-101820 discloses an induction system having the following construction: The induction system is incorporated into an engine having two rows of cylinders. An individual chamber serving as a collecting portion to which upstream ends of the intake runners are connected is provided in correspondence in each cylinder row. Two intake runners extends individually from each cylinder to the respective chambers, one intake runner being short and the other being long. The respective chambers are arranged between the cylinder rows, and a communicating passage is provided with one end thereof connected to one chamber and the other end thereof connected to the other chamber so as to communicate the respective chambers with each other. Further, two types of valves are disposed so as to open the respective intake runners and the communicating passage respectively. The dynamic effects of induction can be made use of in a wide range of engine speed by opening or closing the respective valves in an appropriate manner.
Further, Unexamined Japanese Utility Model Publication No. 60-88062 discloses an engine induction system in which intake runners extending individually from respective cylinders are connected to a convergent portion at upstream end thereof in such a manner that the lengths of the respective intake runners are equal to one another. As a result, the engine induction system demonstrates an improved distributivity of the intake air into the respective cylinders, and improved and uniform dynamic effects in the respective cylinders.
As an induction system for use in a V-shaped engine, Unexamined Japanese Utility Model Publication No. 2-54359 discloses the one in which respective intake runners extending individually from each bank are connected to a connecting portion including a surge tank. The connecting portions provided for the respective two banks are arranged above one of the bank with upstream portions thereof being connected to each other. Further, Unexamined Japanese Patent Publication No. 62-153516 discloses an induction system in which a surge tank having a large volume is horizontally partitioned into an upper chamber and a lower chamber. Upstream ends of respective intake runners extending individually from one bank are connected to the upper chamber while those of respective intake runners extending individually from the other bank being connected to the lower chamber. Upstream ends of the respective chambers are connected to common intake passages, which are connected to each other at a further upstream side.
In the case where the intake runners having the lengths equal to one another as disclosed in Unexamined Japanese Utility Model Publication No. 60-88062 are used in an engine having a plurality of cylinder rows as disclosed in Unexamined Japanese Patent Publication No. 62-101820, the following thing is required. Since there are generally provided many cylinders in such an engine, a connecting portion corresponding to each cylinder row is required to be small. In this case, if the respective small size connecting portions are arranged between the cylinder rows, it causes the connecting portions and the corresponding cylinders to be positioned closer relative to each other, with the result that the intake runners extending from the respective cylinders become shorter.
However, the intake runners cannot be made smaller in diameter thereof since it is necessary to assure a predetermined flow rate of the intake air in the respective cylinders. Accordingly, in the case where the individual intake runners are shortened, an engine speed region in which the maximum resonance effect or inertia effect can be obtained shifts to a higher level. However, there are some cases where such an effective engine speed region exceeds actual engine speed region of an engine used in a general passenger vehicle. In view thereof, it is necessary to assuredly give each intake runners the substantial length. However, this causes the small size connecting portions to be moved upward in view of layout thereof in the induction system where the small size connecting portions are arranged between the cylinder rows. Consequently, the height of the engine increases, which deters the construction of the engine having a smaller size.
On the other hand, if the respective small size connecting portion are arranged above the one of the cylinder rows, the above problem may be solved. However, there is yet room for improvement in making the size of the induction system smaller while making the lengths of the respective intake runners equal to one another and utilizing the dynamic effects in a wide range of engine speed, facilitating an assembling operation of an intake manifold to an engine main body, and the like.
Further, in an induction system in which collecting portions constituted by surge tanks having a relatively large volume and provided for the respective banks are arranged above one of the banks, it is difficult to make the size of the induction system sufficiently small. Also, if an intake manifold including the respective intake runners and connecting portions is integrally formed, it is difficult to perform such operations as to fix the intake manifold to a cylinder head while supporting it. Further, it still remains as a problem to facilitate assembling of the intake manifold to the engine main body.